The present invention relates to the fabrication of optical quality thin films, and more particularly to the low pressure fabrication of such thin films for application in non-linear optical devices and organic light emitting devices.
The field of organic electroluminescence is a rapidly growing technology. Spurred by potential application to displays, organic light emitting devices (OLEDs) are capable of achieving external quantum efficiencies of over 3%, and operational lifetimes on the order of 10,000 hours at video brightness. Both small molecule and polymer-based OLEDs are known, but polymer-based devices have a general advantage of simple and inexpensive fabrication by spin-on deposition techniques. In contrast, small molecule devices are usually fabricated by thermal evaporation in vacuum, which is usually a more expensive process than spin-on deposition. Examples of OLED structures and processing techniques are provided in published PCT application WO 96/19792, incorporated herein by reference.
The use of organic vapor phase deposition (OVPD) has made progress towards the low cost, large scale deposition of small molecular weight organic layers with numerous potential photonic device applications such as displays. The OVPD process is described in U.S. Pat. No. 5,554,220 to Forrest et al.; S. R. Forrest et al., xe2x80x9cIntense Second Harmonic Generation and Long-Range Structural Ordering in Thin Films of an Organic Salt Grown by Organic Vapor Phase Deposition,xe2x80x9d 68 Appl. Phys. Lett. 1326 (1996); and P. E. Burrows et al., xe2x80x9cOrganic Vapor Phase Deposition: a New Method for the Growth of Organic Thin Films with Large Optical Non-linearities,xe2x80x9d 156 J. of Crystal Growth 91 (1995), each of which is incorporated herein by reference.
The OVPD process uses carrier gases to transport source materials to a substrate, where the gases condense to form a desired thin film. The OVPD technique has been used, for example, to deposit films of the optically non-linear organic (NLO) salt, 4xe2x80x2-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), from volatile precursors 4xe2x80x2-dimethylamino-N-methyl-4-stilbazolium iodide (DASI) and methyl p-toluensulfonate (methyltosylate, MT), which are transported by carrier gases to a heated substrate. In this process, DASI thermally decomposes to form 4-dimethylamino-4-stilbazole (DAS), which subsequently reacts with MT to form DAST on the substrate.
Because of its capability for controlled co-deposition of materials with radically different vapor pressures, OVPD is believed to be the only method for the precise stoichiometric growth of multi-component thin films. However, the OPVD process is conducted at atmospheric pressure, and films grown at or near atmospheric pressure are often rough and have non-uniform surface morphologies due to gas phase nucleation and a diffusion-limited growth process.
The present invention makes use of low pressure deposition techniques to produce organic thin films having superior surface properties. In one aspect, the present invention comprises a method for preparing an organic thin film on a substrate, the method comprising the steps of providing a plurality of organic precursors, the organic precursors being in the vapor phase; and reacting the plurality of organic precursors at a sub-atmospheric pressure in the presence of the substrate to form a thin film on the substrate. In another aspect, the present invention includes organic films made by such a method. In yet another aspect, the present invention includes an apparatus designed to facilitate the reaction of organic precursors at sub-atmospheric pressures to form an organic film on a substrate.
One advantage of the present invention is that it provides multi-component organic thin films wherein the amount of each component in such films can be controlled accurately and precisely.
Another advantage of the present invention is that it provides uniform organic thin films having smooth surfaces.
Another advantage of the invention is that it provides a low pressure organic vapor phase deposition method and apparatus for the growth of thin films of organic light emitting materials and optically non-linear organic salts.
Another advantage of the invention is that it provides a low pressure organic molecular beam deposition method and apparatus for the formation of thin films of organic light emitting materials and optically non-linear organic salts.
Yet another advantage of the invention is that it provides a method and apparatus for the uniform deposition of organic materials over large substrate areas.